US 6862197 B2
To demagnetize a transformer (TR) in a single-ended forward DC/DC converter with self-driven synchronized rectifiers (V1, V2) connected across the secondary winding (N2) of the transformer (TR), a diode (D1) is connected in series with a capacitor (C1) across the primary winding (N1) of the transformer. The diode (D1) transfers magnetization energy stored in the transformer to the capacitor (C1) every time a primary switch (V3) of the converter is turned off. To ensure optimum efficiency of the synchronized rectifiers, a discharging circuit is connected to the capacitor (C1) for discharging the magnetization energy stored therein by drawing a DC current (I) from the capacitor (C1) in response to varying input DC voltage such that complete demagnetization of the transformer (TR) always is attained just before turn-on of the primary switch (V3).
1. An arrangement for demagnetization of a transformer (TR) in a single-ended forward DC/DC converter, a primary winding (N1) of the transformer being connected in series with a primary switch (V3) to a source (U1) of varying input DC voltage, a diode (D1) being connected in series with a capacitor (C1) across the primary winding (N1) for transferring magnetization energy from the transformer (TR) to the capacitor (C1) during the off period of the primary switch (V3), and a discharging circuit being connected across the capacitor (C1) in order to dissipate magnetization energy stored therein, characterized in that the discharging circuit comprises means for discharging the capacitor (C1) with such a DC current (I) in response to the varying input DC voltage that complete demagnetization of the transformer (TR) always is attained just before turn-on of the primary switch (V3).
2. The arrangement according to
This application is the U.S. National phase of international application PCT/SE02/00136 filed 25 Jan. 2002 which designates the U.S.
The invention relates generally to forward DC/DC converters and more specifically to an arrangement for demagnetizing, i.e. resetting, a transformer in such converters.
An output filter comprising an inductor L and a capacitor C is connected across V2 to provide an output voltage U2 across the capacitor C in a manner known per se.
A primary winding N1 of the transformer TR is connected with one of its terminals to a (+) terminal 1 of a source of varying input DC voltage U1, and with its other terminal to the drain of a primary switch in the form of a FET V3. The source of V3 is connected to a (−) terminal 2 of the voltage source U1. The gate of V3 is pulse width modulated such that its duty cycle is varied in response to the varying input voltage U1 to keep the output voltage U2 at a desired value. To accomplish this, the actual value of the output voltage U2 is sensed by a voltage regulator 3 and compared to the desired value of the output voltage, that is set in the voltage regulator 3. In response to differences between the actual value and the desired value, the voltage regulator 3 outputs a control signal to a control circuit 4. In response to the control signal, the control circuit 4 in its turn outputs a pulse width modulated control signal to the gate of V3 to vary the duty cycle of V3 such that the actual value of U2 equals the desired value. During the off period of the primary switch V3, the core of the transformer TR has to be reset to discharge the leakage inductance of the transformer TR.
To reset or demagnetize the transformer TR, a so-called snubber circuit is provided in a manner known per se to absorb energy during the off period of V3. The snubber circuit comprises a series circuit of a diode D1 and a capacitor C1 that is connected in parallel with the primary winding N1 and a resistor R1 which is connected to the terminals of the capacitor C1. When V3 is turned off, energy which has been accumulated in the primary winding N1 of the transformer TR is transferred to the capacitor C1 and dissipated by the resistor R1.
The FETs V1 and V2 are both controlled by the transformer TR in such a manner that V1 is on when V3 is on, while V2 is on when V3 is off. Thus, V2 is on when the transformer TR is being reset. At higher input voltages U1, the on periods of V3 will be shorter. Hereby, the transformer TR will be reset more quickly. This will result in a longer so-called dead time, i.e. the time when there is no voltage across the transformer TR. As a consequence, V2 will not have any gate drive during such times. Instead, its body diode that generates more losses, will conduct. Hereby, the efficiency of the converter will be lower. Also, the presence of dead time means that the primary switch V3 is exposed to higher voltage than necessary.
To improve the efficiency that is associated with good timing of the secondary switches, it is possible to control the gate drive of V2 from the primary side of the transformer TR. The disadvantages of such a solution are increased complexity and increased costs.
The object of the invention is to bring about an arrangement for demagnetizing the transformer in a single-ended forward DC/DC converter with self-driven synchronized rectifiers to ensure optimal operation of the synchronized rectifiers and optimal efficiency of the converter.
This is attained by providing the converter with an arrangement for demagnetizing/resetting the transformer such that complete demagnetization of the transformer always is attained just before turn-on of the primary switch.
Hereby optimal operation of the synchronous rectifiers is achieved as well as minimum voltage stress of the primary switch.
The invention will be described more in detail below with reference to the appended drawing on which
To ensure optimum efficiency of the synchronized rectifiers V1, V2 in a single-ended forward DC/DC converter, in accordance with the invention, the converter is provided with an arrangement for demagnetizing/resetting the transformer TR such that complete demagnetization of the transformer always is attained just before turn-on of the primary switch V3. This is called optimum resetting of the transformer TR and is obtained when ton·U1=(1−ton)·UC1, where t,D is the on-time of V3, U1 is the varying input DC voltage, and UC1 is the voltage across the capacitor C1 in the snubber circuit.
The embodiment of the discharging circuit in
With reference to
The purpose of the discharge circuit according to the invention is to reduce the voltage UC1 across the capacitor C1 when the input voltage U1 increases. To accomplish this, in accordance with the invention, C1 is discharged with a discharge current I≈A·U1+B, where A and B are constants.
In the embodiment shown in
In this case, A=R2/(R1·R3) and B=UZ·R2/(R1·R3).
Thus, when the input voltage U1 increases, the discharge current I will increase. At the same time, the on-time of V3 is reduced.
The on-time of V3 lasts from t=0 to t=t1 in FIG. 4. At time t1, the primary switch V3 is turned off. The magnetizing energy in N1, which is kept constant indirectly by the control circuit 4, will be dissipated mainly in the discharging circuit. However a portion of this energy is stored in the stray capacitances of V3, D1 and N1.
When the voltage UN1 across the primary winding N1 of the transformer TR is 0 at time t1, i.e. when the magnetization of N1 ends, the energy in N1 is independent of U1. At this instant, the demagnetization starts.
The stray capacitances loading N1 will be charged, storing some of the magnetization energy. The remaining magnetization energy is stored in C1 and hence dissipated in the discharging circuit according to the invention. The main part of that energy is lost in the transistor T1.
At time t2 in
Consequently, complete demagnetization of the transformer TR is attained just before turn-on of the primary switch V3 at time t3.
Thus, by means of the discharging circuit according to the invention, there will be no dead time.